MARKET PLACE
Stent grafts
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Figure 1: Stent graft to treat aneurysms and dissections in the thoracic aorta (JOTEC GmbH).
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Figure 2: FEA model of the tested stent springs (Endosmart GmbH, Stutensee, Germany). (click image to enlarge) |
Aneurysms in the thoracic aorta are dilations of the vessel wall. The predomi-nant cause is arteriosclerosis and 8–10 new cases arise per 100 000 people annually.1,2 There is an acute risk of rupture and less than 50% of those affected are able to reach a hospital in time for treatment.3 Treatment usually involves insertion of a vessel wall endoprosthe-sis, that is, a stent graft. This separates the flow of blood and a considerable part of the blood pressure from the aneurysm and in this way reduces the risk of rupture. According to EN 14299, Specific Requirements for Arterial Stents, all stent grafts mu st be subjected to fatigue testing for CE-marking.4 At least six stent grafts must be demonstrated to withstand a physiological variation in stress of 380 million cycles. This corre-sponds to an implantation period of 10 years. During use, the thoracic aortic stent graft is exposed to an essentially oscillating radial stress in the thoracic area of the aorta, whereas a stent graft in the abdominal area is subject to more complex axial and flexoral stress.5
Materials and method
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Table 1: Comparison MISES reference stress and strain from finite element analysis.
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Table 2: Apparatus-related parameters.
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The aortic stent graft being tested was developed by JOTEC GmbH (Hechingen, Germany) (Figure 1). The stent graft consists of a polyester tube into which zigzag-shaped stent springs made of a super elastic nickel-titanium (NiTi) alloy are attached (Figure 2).The parameters selected for the fatigue test are based on investigations of the aorta in “healthy” young people. This group represents the most serious physiological stress case for a stent graft. Young people can also be patients because they frequently suffer traffic accidents that could lead to traumatic aorta lesions. Their aortas still exhibit a particularly high degree of elasticity and therefore show large diameter/pressure changes. This characteristic is continually diminishing in older people as a result of the natural decomposition of elastin in the vessel walls and arteriosclerotic processes.6 Simultaneous measurements of aortal pressure diameter curves in 48 locally anaesthetised test subjects provided data to establish the physiological parameters.7 These data were originally taken to examine the influence of active and passive smoking on thoracic aorta compliance. For aorta diameters (d) of 20–26 mm, diameter fluctuations (Δd) of 1.3–2.1 mm were measured. Because no stent graft was taken into consideration for these values, supplementary trials and calculations were performed.8 The results reveal a wall pressure of approximately 60 mmHg in the event of oversizing the diameter of the test stent graft by 6 mm. This pressure is transposed into physiological pressure, which leads to a quasi-hypertonically stressed aorta with correspondingly reduced compliance. The diameter fluctuations with stent grafts are therefore reduced to Δd of 0.6– 1.6 mm.8 EN 14299 allows fatigue testing to be limited to one stent graft configuration; however, the selected configuration must be one with the highest potential for fatigue failure. Experiences of competitors9 and previous tests have shown that, of all possible configurations, zigzag radii have the highest sensitivity to mechanical failure. Already during manufacture of the stent springs, the source material, a straight NiTi wire, undergoes great stress through thermal fixation. The initial wire experiences the greatest deformation in the 24F deployment catheter, if the diameter of the stent graft is 28 mm when stent spring heights and diameters are set in relationship with each other. This relationship also applies to the static radial stress, if the stent graft is to ensure a sufficient sealing and migration safety on the anchoring zones in an aorta.
Finite element analysis (FEA) using the hysteresis data of the NiTi wires, stent spring geometry and static and dynamic compression conditions during implantation, resulted in MISES reference stress and strain (Table I).10 This stress has no direction, but gives an analytical measure of internal loading.
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Figure 3: Diagram of one compartment of the test apparatus with a loaded stent graft (NMI).
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Figure 4: Sample: 58179-ST-21;
interior view of the stent graft.
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Stent grafts with eight stent springs were investigated. In the course of the fatigue tests, 48 stent springs, each with 10 zigzag radii, equating to 480 (48310) potential rupture places were tested. The stents were inserted into polyurethane (PU) tubes (see position 6 in Figure 3) with the help of a 24 F deployment catheter. The tubes were positioned rotationally like “spokes” in a wheel and assembled in a vibration test apparatus (Carl Schenk AG, Darmstadt, Germany).11 A pulsating pressure from the hub was applied to the open end of the tubes. The vibration test apparatus presses against a stainless steel bellow when the bottom stamp vibrates. The silicone oil filling of the stainless steel bellow transfers the hydraulic pressure fluctuations through an elastic separating membrane onto the transfer medium inside the hub (Figure 3). The transfer medium for the highly frequent pressure pulses and for the simulation of the blood is a physiological saline solution set to body temperature (37°C). Apparatus-related parameters are shown in Table II.
To verify the setting parameters during the 50 Hz vibration, high-speed pictures were taken and evaluated using a SpeedCam Visario video system (Weinberger, Karlsruhe, Germany). The radially acting pressure pulses in the stent graft were photographed at a rate of 20 pictures per cycle (that is, 1000 pictures per second). In this way the actual amplitude, course, shape and loss could be observed. This enabled the external machine settings of the test system to be precisely calibrated and adjusted. After approximately 100 million load cycles each, the inner side of the stent grafts was inspected using an endoscope (30° observation angle, Karl Storz, Tuttlingen, Germany).
If the fatigue is accelerated to 50 Hz, test durations of approximately 3 months are required to be able to make statements about the material fatigue of the stent grafts. Material changes in the stent grafts, for example, cracks in the tissue attachment and tissue, and damage to the X-ray markers and to the NiTi wires were documented.
Results
The 50 Hz fatigue testing system is able to test six stent grafts in 88 days. The stent grafts being tested were visually inspected, externally and internally, using an endoscope. After 380 million stress cycles the fixture of the X-ray marker was still intact. The puncture points of the longitudinal seam were not extended, the threads for attaching the stent springs and the NiTi wires were undamaged.
Summary
The aortic stent graft was fatigue tested in the course of CE-approval in line with standard EN 14299. Evaluation of literature and supplementary calculations resulted in physiological fluctuations in diameter of an aorta with a stent graft of Δd of 0.6–1.6mm. With these values, periodic load equivalents of radial stress were set in the framework of the newly developed fatigue test. FEM analyses with the tension/expansion hystereses of the NiTi wires, the stent spring geometry and the static and dynamic compression states during implantation provide information that quantitatively supplements the fatigue test. After 380 million cycles no visible damage as a result of material fatigue was observed on the stent grafts tested. Thus, it can be stated that according to current standards, the stent graft in question can be considered to be fatique resistant under physiological worst case conditions.
References
1. L.K. Bickerstaff et al., “Thoracic Aortic Aneurysms: A Population Based Study,” Surgery, 92, 1103–1108, 1982.
2. H.R. Sørenson, H. Olsen, “ Ruptured and Dissecting Aneurysms of the Aorta: Incidence and Prospects of Surgery,“ Acta. Chir. Scand., 128, 644–50, 1964.
3. G. Johansson, U. Markstrom, J. Swedenborg, “Ruptured Thoracic Aortic Aneurysms: A Study of Incidence and Mortality Rates,” J. Vasc. Surg., 21, 6, 985–988, 1995.
4. EN 14299, “Non Active Surgical Implants – Particular Requirements for Cardiac and Vascular Implants – Specific Requirements for Arterial Stents,” May 2004.
5. Z. Li, C. Kleinstreuer, M. Farber, “Computational Analysis of Biomechanical Contributors to Possible Endovascular Graft Failure,” Biomech. Model. Mechanbiol., 4, 4, 221–234, 2005.
6. S.S. Najjar, A. Scuteri, E.G. Lakatta, “Arterial Aging: Is It An Immutable Cardiovascular Risk Factor?” Hypertension, 46, 3, 454–462, 2005.
7. C. Stefanadis et al., “Unfavorable Effects of Passive Smoking on Aortic Function in Men,” Ann. Int. Med., 128, 6, 426–434, 1998.
8. R. Kaufmann, “Endoprosthesis For Treatment of Thoracic Aortic Aneurysms: Mechanical Aspects of Graft Fixation,” Unpublished, presentation at the Industrial Colloquium at RWTH Aachen University, 23 May 2003.
9. B. James, S. Murray, S. Saint, “Fracture Characterisation in Nitinol,” SMST–2003: Proceedings of the International Conference on Shape Memory and Superelastic Technologies, eds. A.R. Pelton, T. Duerig, 321–329, 2004.
10. F. Ziegler, “Mechanics of Solids and Fluids,” 2nd ed., Springer-Verlag New York Inc., 1995.
11. B. Schroeder, “Structure and Results of the E-Vita Fatigue Test at the Natural and Medical Science Institute,” Unpublished, presentation 7 May 2004.
Acknowledgements
The authors are grateful to Andrea Karl, Philipp Kammerlohr and Michael Quellmalz (NMI), and Stefan Derkvist and Rainer Lesmeister (JOTEC GmbH).
Dr Bernhard Schröder is Head Surface Technology at NMI Natural and Medical Sciences Institute at the University of Tuebingen, Markwiesenstrasse 55, D-72770 Reutlingen, Germany, tel. +49 7121 51530 52, e-mail: schroeder@nmi.de, www.nmi.de
Dr Ralf Kaufmann is Head of the Stent Development Department at JOTEC GmbH, Lotzenäcker 23, D-72379 Hechingen, Germany, tel. +49 7471 922 181, e-mail: ralf.kaufmann@jotec.net, www.jotec.net
